You stated :
"It's a matter of common English Usage."
Not correct. Look up any dictionary. "Alien" is not a synonym for "Astronaut."
"Most people are inclined to think of technologically advanced (usually) Extra-terrestrials when the word 'aliens' is mentioned."
Certainly not before the 20th Century. The term alien has been in use for thousands of years and with no reference to technologically advanced beings.
"Personally, I blame 'Hollywood' - and Sigourney Weaver in particular."
Not being a fan of silly Hollywood movies, I will have to take your word on that.
"So I am not entirely unsympathetic towards your grammar-based stance over 'Ancient Alien v Ancient Astronaut'."
Not sure of your education in the use of the English language, but my comments are not based on some grammatical faux pas.
It's pretty simple really. Aliens are not Astronauts.